The present invention is related generally to an electrolytic process and apparatus for recovering copper and other metals. The process and apparatus of the present invention are useful in both electrowinning and electrorefining. The present invention is specifically directed to series electrowinning and electrorefining.
In order to facilitate understanding the present invention, a brief discussion relative to electrorefining and electrowinning in a series cell follows. FIG. 1 is a diagram showing the general arrangement upon which a series cell in accordance with the present invention is operated in a refining mode. The fundamental characteristic of any series cell is that a series of bipolar electrodes, 10, 12, 14, 16 and 18 which are unconnected to any electrical circuit, is located between an anode and cathode pair. When the circuitry of the cell is completed, electric current passes from the anode 20 through all the bipolar electrodes in a series to the cathode 22 as is shown by arrow 24. When the cell is in operation, metal is plated on that surface of each bipolar electrode which faces the anode, i.e. cathodic surfaces 26, 28, 30, 32 and 34; metal is etched away from the surface of the bipolar electode facing the cathode i.e. anodic surfaces 36, 38, 40, 42 and 44. Of course, metal is also deposited on cathode face 46 and removed from the immersed surface of anode 20. At this point, it should be noted that a cell constructed in accordance with the present invention can contain more than the five bipolar electrodes shown in FIG. 1. The number of such bipolar electrodes is a detail which is well within the skill of those in this art.
In connection with the bipolar electrodes used in the cell of the present invention, it should be noted that the cell of the present invention can be operated with a novel composite bipolar electorde such as those shown in FIG. 1 by reference numerals 10, 14 and 18. The concept involved in the composite bipolar electrode structure is to employ a base sheet or substrate of electrochemically suitable material such as titanium or other "valve" metal or stainless steel (S.S.) and affix to it, on one side, a layer of refinable anode material such as copper. Further details of such bipolar electrodes appear below.
Of course, the cell of the present invention can employ conventional bipolar electrodes such as copper slabs or sheets 12 and 16. Due to the electrochemical action within the cell, metal is etched away from the surface of sheets 12 and 16 facing the cathode and is deposited on the surface of sheets 12 and 16 facing the anode so that after operating for a period of time, the positions of sheets 12 and 16 shift to the locations shown by the dotted line pairs in FIG. 1.
When a series electrodeposition cell is employed for electrorefining of a metal such as copper, the anode is a slab of that metal, and the bipolar electrodes can be sheets of the same metal or the novel composite structures described above. A series cell would normally include only one style of bipolar electrode.
When a series cell is used for electrowinning, the anode is an insoluble anode formed of metal such as lead or lead alloy or of precious metal clad titanium, or the like. The bipolar electrodes are then constructed of similarly insoluble materials that allow oxygen evolution at their anodic face while permitting the deposited metal to be stripped from their cathodic face. With both modes, winning and refining, of series electrodeposition, the end cathode may be a starter sheet of the metal to be deposited or, preferably for this invention, a rigid nonretentive blank of stainless steel or titanium or the like.
There are many advantages in utilizing series electrodeposition cells for both electrowinning and electrorefining. Principal among them are the use of much lower cell currents than in the parallel system of electrodeposition and the elimination of electrical contacts to all but the end electrodes of a cell. The advantages of the series system become even greater at very high current densitites, where in the parallel system low-resistance clamps are required for every electrode, thus complicating the operation and rendering the attainment of the desired close spacing very difficult.
One disadvantage of utilizing prior art electrodeposition series cells for electrowinning or electrorefining is that the prior art cells were not capable of being efficiently utilized at high current densitites; that is, in the case of copper, current densities in excess of 17-20 amps per square feet. In the present invention high current density is advantageously employed to reduce plant size and metal inventory. In connection with the foregoing, the term "current density" is the ratio of current in amperes to the area of cathode in square feet and is expressed in ASF units. Of course, it is well known in this art that an increase in current density decreases the time required for a given amount of metal deposition.
One of the problems associated with prior art electrodeposition series cells is the phenomenon known as "current bypass". When current bypass occurs, a portion of the current does not pass through the bipolar electrodes but passes under or around the bipolar electrodes from the anode to the cathode as is shown by arrow 50. Of course, any current that passes from the anode to the cathode bypassing the bipolar electrodes will not contribute toward plating metal on the bipolar electrodes. Accordingly, any efficient series cell must have some means in it for reducing current bypass.
The most significant problem associated with the prior art means for blocking out current bypass is that the means for blocking out the current bypass do not allow for sufficient convection of the electrolyte. In connection with the foregoing, convection is necessary to prevent stagnation of the electrolyte, which results in depletion of the depositing metal ions at cathodic surfaces and may result in passivation of soluble anode surfaces.